MAX5038/MAX5041
Dual-Phase, Parallelable, Average Current-Mode
Controllers
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V
IN
and V
CC
The MAX5038/MAX5041 accept a wide input voltage
range of +4.75V to +5.5V or +8V to +28V. All internal
control circuitry operates from an internally regulated
nominal voltage of +5V (V
CC
). For input voltages of +8V
or greater, the internal V
CC
regulator steps the voltage
down to +5V. The V
CC
output voltage regulates to +5V
while sourcing up to 80mA. Bypass V
CC
to SGND with
4.7µF and 0.1µF low-ESR ceramic capacitors for high-
frequency noise rejection and stable operation (Figures 1
and 2).
Calculate power dissipation in the MAX5038/MAX5041
as a product of the input voltage and the total V
CC
reg-
ulator output current (I
CC
). I
CC
includes quiescent cur-
rent (I
Q
) and gate drive current (I
DD
):
P
D
= V
IN
x I
CC
I
CC
= I
Q
+ f
SW
x (Q
G1
+ Q
G2
+ Q
G3
+ Q
G4
)
where, Q
G1
, Q
G2
, Q
G3,
and Q
G4
are the total gate
charge of the low-side and high-side external
MOSFETs, I
Q
is 4mA (typ), and f
SW
is the switching fre-
quency of each individual phase.
For applications utilizing a +5V input voltage, disable
the V
CC
regulator by connecting IN and V
CC
together.
Undervoltage Lockout (UVLO)/
Power-On Reset (POR)/Soft-Start
The MAX5038/MAX5041 include an undervoltage lock-
out with hysteresis and a power-on reset circuit for con-
verter turn-on and monotonic rise of the output voltage.
The UVLO threshold is internally set between +4.0V
and +4.5V with a 200mV hysteresis. Hysteresis at
UVLO eliminates “chattering” during startup.
Most of the internal circuitry, including the oscillator,
turns on when the input voltage reaches +4V. The
MAX5038/MAX5041 draw up to 4mA of current before
the input voltage reaches the UVLO threshold.
The compensation network at the current error ampli-
fiers (CLP1 and CLP2) provides an inherent soft-start of
the output voltage. It includes a parallel combination of
capacitors (C28, C30) and resistors (R5, R6) in series
with other capacitors (C27, C29) (see Figures 1 and 2).
The voltage at CLP_ limits the maximum current avail-
able to charge output capacitors. The capacitor on
CLP_ in conjunction with the finite output-drive current
of the current-error amplifier yields a finite rise time for
the output current and thus the output voltage.
Internal Oscillator
The internal oscillator generates the 180° out-of-phase
clock signals required by the pulse-width modulation
(PWM) circuits. The oscillator also generates the 2V
P-P
voltage ramp signals necessary for the PWM compara-
tors. Connect CLKIN to SGND to set the internal oscillator
frequency to 250kHz or connect CLKIN to V
CC
to set the
internal oscillator to 500kHz.
CLKIN is a CMOS logic clock input for the phase-
locked loop (PLL). When driven externally, the internal
oscillator locks to the signal at CLKIN. A rising edge at
CLKIN starts the ON cycle of the PWM. Ensure that the
external clock pulse width is at least 200ns. CLKOUT
provides a phase-shifted output with respect to the ris-
ing edge of the signal at CLKIN. PHASE sets the
amount of phase shift at CLKOUT. Connect PHASE to
V
CC
for 120° of phase shift, leave PHASE unconnected
for 90° of phase shift, or connect PHASE to SGND for
60° of phase shift with respect to CLKIN.
The MAX5038/MAX5041 require compensation on
PLLCMP even when operating from the internal oscillator.
The device requires an active PLL in order to generate
the proper clock signal required for PWM operation.
Control Loop
The MAX5038/MAX5041 use an average current-mode
control scheme to regulate the output voltage (Figures
3a and 3b). The main control loop consists of an inner
current loop and an outer voltage loop. The inner loop
controls the output currents (I
PHASE1
and I
PHASE2
)
while the outer loop controls the output voltage. The
inner current loop absorbs the inductor pole reducing
the order of the outer voltage loop to that of a single-
pole system.
The current loop consists of a current-sense resistor
(R
S
), a current-sense amplifier (CA_), a current-error
amplifier (CEA_), an oscillator providing the carrier
ramp, and a PWM comparator (CPWM_). The precision
CA_ amplifies the sense voltage across R
S
by a factor
of 18. The inverting input to the CEA_ senses the CA_
output. The CEA_ output is the difference between the
voltage-error amplifier output (EAOUT) and the gained-
up voltage from the CA_. The RC compensation net-
work connected to CLP1 and CLP2 provides external
frequency compensation for the respective CEA_. The
start of every clock cycle enables the high-side drivers
and initiates a PWM ON cycle. Comparator CPWM_
compares the output voltage from the CEA_ with a 0 to
+2V ramp from the oscillator. The PWM ON cycle termi-
nates when the ramp voltage exceeds the error voltage.
(1)
(2)
